Label film for deep drawing methods

ABSTRACT

The invention relates to the use of a bixailly oriented film having a porous layer, which contains propylene polymer and at least one β-nucleating agent and whose microporosity is generated by converting β-crystalline polypropylene during stretching of the film, for labeling containers during a drawing process.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2005/010746 filed Oct. 5, 2005, which claims the benefit ofGerman application 10 2004 048 811.8 filed Oct. 7, 2004.

The present invention relates to the use of a biaxially orientedpolypropylene film as an in-mold label in deep drawing.

Label films comprise an extensive and technically complex field. Onedifferentiates between different label technologies, which basicallyvary in regard to the process conditions and necessarily have differenttechnical requirements for the label materials. All labeling processesshare the feature that visually appealing labeled containers must resultas the final product, in which good adhesion to the labeled containermust be ensured.

Greatly varying techniques are used for applying the label in thelabeling methods. One differentiates between self-adhesive labels,wraparound labels, shrink labels, in-mold labels, patch labels, etc. Theuse of a film made of thermoplastic as a label is possible in all ofthese various labeling methods.

Various technologies are also differentiated for in-mold labeling, inwhich various method conditions are applied. All in-mold labelingmethods share the feature that the label participates in the actualmolding method of the container and is applied during this method.However, greatly varying molding methods are used for this purpose, suchas injection molding methods, blowmolding methods, and deep-drawingmethods.

In the injection molding method, a label is laid in the injection moldand a molten plastic is injected behind it. The label bonds to theinjection-molded part due to the high temperatures and pressures andbecomes an integral, nonremovable component of the molded part.

For example, tubs and covers of ice cream or margarine tubs are producedaccording to this method.

For this purpose, individual labels are taken from a stack or cut tolength from a roll and laid in the injection mold. The mold is designedin such a way that the melt flow is injected behind the label and thefront side of the film presses against the wall of the injection mold.During the injection, the hot melt bonds to the label. After theinjection, the mold opens, the molded part having the label is ejectedand cools. In the product, the label must adhere to the containerwithout wrinkles and in a visually perfect way.

During the injection, the injection pressure is in a range from 300 to600 bar. The plastics used have a melt-flow index of approximately 40g/10 minutes. The injection temperatures are a function of the plasticused. In many cases, the mold is additionally cooled to avoid stickingof the molded part to the mold.

In deep drawing, unoriented thick plastic slabs, usually cast PP or PS(polystyrene) at a thickness of approximately 200 μm, are heated anddrawn or pressed using vacuum or ram tools in a corresponding mold. Theindividual label is also inserted into the mold and bonds during themolding process to the actual container in this case. Significantlylower temperatures are applied, so that the adhesion of the label to thecontainer may be a critical factor. Good adhesion must be ensured evenat these low processing temperatures. The processing speeds of thismethod are lower than in injection molding.

Direct in-mold labeling is also possible in the blowmolding ofcontainers or hollow bodies. In this method, a melt tube is extrudedvertically downward through a tubular die. A vertically divided moldmoves together and encloses the tube, which is pinched closed at thelower end. A blow pin is inserted at the upper end, by which the openingof the molded part is implemented. Air is supplied to the hot melt tubevia the blow pin, so that it expands and presses against the inner wallsof the mold. The label must bond to the viscous plastic of the melt tubein this case. The mold is subsequently opened and the residue is cut offat the molded opening. The molded and labeled container is ejected andcools.

In this blowmolding method, the pressure during inflation of the melttube is approximately 4-15 bar and the temperatures are significantlylower than in injection molding. The plastic materials have a lower MFIthan in injection molding in order to form a dimensionally stable melttube and therefore behave differently during the cooling process thanthe low viscosity materials for injection molding.

In principle, films made of thermoplastics may also be used for labelingthe containers during molding in deep drawing. For this purpose, thefilms must have a selected property profile to ensure that the labelfilm and the deep-drawn molded body fit against one another withoutbubbles during the deep drawing and bond to one another.

The adhesion of the label to the container is frequently flawed, becausecomparatively lower temperatures and pressures are used in deep drawingthan in injection molding or blowmolding methods. Furthermore, similarlyto blowmolding, air inclusions arise between the label and thecontainer, which impair the appearance of the labeled container and alsothe adhesion. Therefore, labels for deep drawing applications areequipped with special adhesion layers which ensure good adhesion to thecontainer. Coextruded, low-sealing cover layers or special adhesivelayers are used for this purpose.

A film of this type is described, for example, in WO 02/45956. The coverlayer of this film has improved adhesive properties in relation togreatly varying materials. The cover layer contains a copolymer orterpolymer made of an olefin and unsaturated carboxylic acids or theiresters as the main component. It is described that this film may also beused as a label in deep drawing because of the improved adhesion.

EP 0 865 909 describes the use of “microvoided” films for labels. Thefilm contains a β-nucleating agent, by which an increased proportion ofβ-crystalline polypropylene is generated in the precursor film uponcooling of the melt film. Upon stretching of the precursor film,“microvoids” are generated. According to the description, the film hasgood printability.

WO 03/091316 describes the use of a biaxially oriented microporous filmwhich contains a propylene polymer and at least one β-nucleating agentand whose microporosity is generated by converting β-crystallinepolypropylene during stretching of the film. According to thedescription, this film may advantageously be used as a label inblowmolding.

The object of the present invention is to provide a label film which maybe used in the deep-drawing method and which has good adhesion inrelation to the container and does not have any air inclusions.

The object on which the present invention is based is achieved by theuse of a biaxially oriented film having a microporous layer, whichcontains polypropylene and β-nucleating agent, and whose microporosityis generated by converting β-crystalline polypropylene during stretchingof the film, for labeling containers in deep drawing.

It has been found that a film having a microporous layer may be usedoutstandingly in deep drawing as a label and no bubbles or airinclusions occur in the special method conditions of the deep-drawingmethod if this microporosity is generated indirectly by β-nucleatingagents. These structures generated in this way differ significantly fromthose of typical vacuole-containing films.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a (top view) and 1 b (cross-section) show the porous layeraccording to the present invention is gas-permeable and displays anopen-pored network structure.

FIGS. 2 a and 2 b show the typical structure of a vacuole-containinglayer made of thermoplastic polymer and incompatible fillers incross-section (2 a) and in a top view (2 b).

FIG. 3 shows various embodiments of the deep-drawing methods

FIG. 4 shows the methods for deep drawing.

A DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2 a and 2 b show the typical structure of a vacuole-containinglayer made of thermoplastic polymer and incompatible fillers incross-section (2 a) and in a top view (2 b). Due to the incompatibilityof the vacuole-initiating particles, cracks occur between the surface ofthe particle and the polymer matrix during stretching, and a closed,air-filled cavity arises, which is referred to as a vacuole. Thesevacuoles are distributed over the entire layer and reduce the density ofthe film, and/or the layer. These films still display a good barriereffect in relation to water vapor, for example, because the vacuoles areclosed and the structure as a whole is not permeable.

In contrast to this, the porous layer according to the present inventionis gas-permeable and displays an open-pored network structure, asvisible from FIGS. 1 a (top view) and 1 b (cross-section). Thisstructure arises not due to incompatible fillers, but rather accordingto an entirely different technical method.

The microporous layer contains polypropylene and β-nucleating agent.This mixture of polypropylene with β-nucleating agent is first melted inan extruder as usual in film production and extruded through a sheet dieas a melt film onto a cooling roll. The β-nucleating agent encouragesthe crystallization of β-crystalline polypropylene during cooling of themelt film, so that an unstretched precursor film having a highproportion of β-crystalline polypropylene arises. The temperature andstretching conditions may be selected during the stretching of thisprecursor film in such a way that the β-crystallite converts into themore thermally stable alpha crystallite of the polypropylene.

Because the density of the β-crystallite is lower, this conversion isaccompanied by volume shrinkage in this area, resulting in thecharacteristic porous structure in connection with the stretchingprocess, similarly to a torn open network. The film externally appearswhite and opaque, even if it does not contain pigments or fillers.

Both methods are known per se in the prior art. Surprisingly, it hasbeen found that a film having a porous layer does not have an orangepeel effect or bubbling if it is used as a label film in deep-drawingmethods and has surprisingly good adhesion in relation to the container.Opaque films having a vacuole-containing layer result in the undesiredorange peel effect and bubbling as labels in deep-drawing methods.Surprisingly, the adhesion of the film having a microporous layer issignificantly improved in relation to films made of polypropylene havinga vacuole-containing structure. In particular, it is very surprisingthat the fibrillated special structure of the microporous layer has apositive influence on the adhesion strength during deep drawing.According to the current knowledge of those skilled in the art, adhesionis primarily determined by the properties of the polymers of the layerwhich is in contact with the container, for example, a lower meltingpoint or a modification of the polymers contributes to improvedadhesion.

The composition of the microporous layer, also referred to as a layer inthe following, will now be described in greater detail. The microporouslayer contains a propylene homopolymer and/or a propylene blockcopolymer, possibly additionally other polyolefins, and at least oneβ-nucleating agent, as well as possibly additional typical additives,such as stabilizers, neutralization agents, lubricants, antistaticagents, and pigments in the particular effective quantities. In general,additional incompatible vacuole-initiating fillers such as calciumcarbonate or polyesters, like PET or PBT, are dispensed with, so thatthe layer generally contains less than 5 weight-percent, preferably 0 toat most 1 weight-percent, of these vacuole-initiating fillers. Smallquantities of this type may reach the layer via the incorporation ofreclaimed film.

In general, the microporous layer contains at least 70→100weight-percent, preferably 80 to 99.95 weight-percent, in particular 90to 97 weight-percent of a propylene homopolymer and/or propylene blockcopolymer and 0.001 to 5 weight-percent, preferably 0.1 to 3weight-percent, of at least one β-nucleating agent, each in relation tothe weight of the layer (the remainder is other polyolefins and/or thecited additives).

Suitable propylene homopolymers contain 80 to 100 weight-percent,preferably 90 to 100 weight-percent propylene units and have a meltingpoint of 140° C. or higher, preferably 150 to 170° C., and generally amelt-flow index of 0.5 to 10 g/10 minutes, preferably 2 to 8 g/10minutes, at 230° C. and a force of 2.16 kg (DIN 53735). Isotacticpropylene homopolymers having an atactic component of 15 weight-percentor less represent preferred propylene polymers for the layer, isotacticpropylene homopolymer being especially preferred.

Suitable propylene block copolymers contain predominantly, i.e., morethan 50 weight-percent, preferably 70 to 99 weight-percent, inparticular 90 to 99 weight-percent propylene units. Suitable comonomersin corresponding quantities are ethylene, butylene, or higher alkenehomologs, among which ethylene is preferred. The melt-flow index of theblock copolymers is in a range from 1 to 15 g/10 minutes, preferably 2to 10 g/10 minutes. The melting point is above 140° C., preferably in arange from 150 to 165° C.

The specified weight percentages relate to the particular polymer.

Mixtures made of propylene homopolymer and propylene block copolymercontain these two components in arbitrary mixture ratios. The ratio ofpropylene homopolymer to propylene block copolymer is preferably in arange from 10 to 90 weight-percent to 90 to 10 weight-percent,preferably 20 to 70 weight-percent to 70 to 20 weight-percent. Mixturesof this type made of homopolymer and block copolymer are especiallypreferred and improve the appearance of the microporous layer.

If necessary, the porous layer may contain other polyolefins in additionto the propylene homopolymers and/or propylene block copolymers. Theproportion of these other polyolefins is generally less than 30weight-percent, preferably in a range from 1 to 20 weight-percent. Otherpolyolefins are, for example, random copolymers of ethylene andpropylene having an ethylene content of 20 weight-percent or less,random copolymers or propylene with C₄-C₈ olefins having an olefiniccontent of 20 weight-percent or less, terpolymers of propylene,ethylene, and butylene having an ethylene content of 10 weight-percentor less and having a butylene content of 15 weight-percent or less, orpolyethylenes, such as HDPE, LDPE, VLDPE, MDPE, and LLDPE.

In principle, all known additives which encourage the formation ofβ-crystals upon the cooling of a polypropylene melt are suitable as theβ-nucleating agent for the microporous layer. β-nucleating agents ofthis type, and also their mode of operation in a polypropylene matrix,are known per se in the prior art and are described in detail in thefollowing.

Various crystalline phases of polypropylenes are known. During thecooling of a melt, the α-crystalline PP predominantly forms, whosemelting point is approximately 158-162° C. A small component ofβ-crystalline phase may be generated during cooling by a specifictemperature control, which has a significantly lower melting point, at148-150° C., than the monocline α-modification. Additives are known inthe prior art which result in increased proportion of the β-duringcrystallization of the polypropylene, such as γ-quinacridone,dihydroquinacridine, or calcium salts of phthalic acid.

For the purposes of the present invention, highly active β-nucleatingagents are preferably used in the porous layer, which generate aβ-proportion of 30-90%, preferably 50-80%, upon cooling of the meltfilm. A two-component nucleation system made of calcium carbonate andorganic dicarboxylic acids is suitable for this purpose, for example,which is described in DE 3610644, to which reference is hereby expresslymade. Calcium salts of dicarboxylic acids are especially advantageous,such as calcium pimelate or calcium suberate, as described in DE4420989, to which reference is also expressly made. The dicarboxamidesdescribed in EP-0557721, in particular N,N-dicyclohexyl-2,6-naphthalenedicarboxamides, are also suitable β-nucleating agents.

In addition to the nucleating agents, maintaining a specific temperaturerange and the dwell time of the melt film at these temperatures as theextruded melt film is drawn off are important for achieving a highproportion of β-crystalline polypropylene in the precursor film. Theextruded melt film is preferably cooled at a temperature of 60 to 130°C., in particular 80 to 120° C. Slow cooling also encourages the growthof the β-crystallite, therefore, the drawing-off speed, i.e., the speedat which the melt film runs over the first cooling roll, is to be slow.For a given configuration of drawing-off rolls, it may be ensured viathe drawing-off speed that the film cools slowly to the particulartemperature, and/or is held at this temperature for a sufficiently longtime. In general, dwell times of 10 seconds to several minutes arepossible. Longer dwell times of over 3 minutes are technically possibleand increase the β-crystalline proportion in a way advantageous per se,but the production process becomes very slow and thus uneconomical usinga process control of this type.

Therefore, the dwell time is preferably 15 to 120 seconds. Thedrawing-off speed is preferably less than 25 m/minute, in particular 1to 20 m/minute. The higher the achieved proportion of β-crystals in theprecursor film, the simpler it is to achieve the net-like porousstructure by stretching, in general, with uniform method conditions,greater porosities are achieved the higher the β-proportion in theprecursor film.

Especially preferred embodiments contain 0.001 to 5 weight-percent,preferably 0.05 to 0.5 weight-percent, in particular 0.1 to 0.3weight-percent calcium pimelate or calcium suberate in the microporouslayer made of propylene polymer.

In general, the microporous label film is single-layered and onlycomprises the microporous layer. However, it is obvious that thissingle-layered film may possibly be provided with a printing or acoating before it is used as a label film in deep drawing. Of course,the surface of the porous layer is in contact with the container and theprinting or coating forms the exterior of the label with multilayerfilms of this type. For such single-layered embodiments, the thicknessof the film, i.e., the porous layer, is in a range from 20 to 150 μm,preferably 30 to 100 μm.

The microporous layer may possibly be provided on the exterior with acorona, flame, or plasma treatment to improve the adhesion in relationto printing inks or coatings.

The density of the microporous layer is generally in a range from 0.2 to0.80 g/cm³, preferably 0.3 to 0.65 g/cm³, a density of less than 0.6g/cm³ being preferred. Surprisingly, it has been found that anespecially low density does not result in an increase of the orange peeleffect, as in vacuole-containing, opaque films. Relevant publicationsteach, in regard to vacuole-containing, opaque films, that too low adensity due to voiding which is too strong results in an increasedorange peel effect. Surprisingly, this is not the case for porous films.The density may be reduced to extremely low values of less than 0.5g/cm³ and the film may nonetheless be applied perfectly in deep drawingwithout a disturbing orange peel effect occurring.

In a further embodiment, the microporous layer may be provided with afurther cover layer, the microporous layer facing toward the containerin the use according to the present invention of this multilayeredembodiment and bonding to the molded body during deep drawing.Accordingly, the additional cover layer forms the exterior of the label.The additional cover layer may be applied by laminating the porous layerwith a further film. It is preferably a coextruded cover layer. In thesemultilayered embodiments, the thickness of the microporous layer is atleast 20 μm, the thickness of the porous layer is preferably in a rangefrom 25 to 100 μm, in particular 30 to 50 μm. The thickness of thiscover layer is generally in a range from 0.5-5 μm, preferably 1-3 μm.

The possibly coextruded cover layer generally contains at least 70weight-percent, preferably 75 to <100 weight-percent, particularly 90 to98 weight-percent of a polyolefin, preferably a propylene polymer andpossibly further typical additives such as neutralization agents,stabilizers, antistatic agents, lubricants, e.g., fatty acid amides orsiloxanes or antiblocking agents in the particular effective quantities.

The propylene polymer of the cover layer is, for example, a propylenehomopolymer, as already described above for the porous layer, or acopolymer made of propylene and ethylene or propylene and butylene orpropylene and another olefin having 5 to 10 carbon atoms. For thepurposes of the present invention, terpolymers of ethylene and propyleneand butylene or ethylene and propylene and another olefin having 5 to 10carbon atoms are also suitable for the cover layer. Furthermore,mixtures or blends made of two or more of the cited copolymers andterpolymers may be used.

Random ethylene-propylene copolymers and ethylene-propylene-butyleneterpolymers are preferred for the cover layer, in particular randomethylene-propylene copolymers having an ethylene content of 2 to 10weight-percent, preferably 5 to 8 weight-percent, or randomethylene-propylene-butylene-1 terpolymers having an ethylene content of1 to 10 weight-percent, preferably 2 to 6 weight-percent and abutylene-1 content of 3 to 20 weight-percent, preferably 8 to 10weight-percent, each in relation to the weight of the copolymer orterpolymer.

The random copolymers and terpolymers described above generally have amelt-flow index of 1.5 to 30 g/10 minutes, preferably 3 to 15 g/10minutes. The melting point is in the range from 105° C. to 140° C. Theblends made of copolymers and terpolymers described above have amelt-flow index of 5 to 9 g/10 minutes and a melting point of 120 to150° C. All melt-flow indices specified above were measured at 230° C.and a force of 2.16 kg (DIN 53735).

The surface of this cover layer may possibly be provided with a corona,flame, or plasma treatment to improve the printability. The density ofthe film is only insignificantly increased by the nonporous cover layer,which also does not contain any vacuoles, in relation to single-layeredembodiments and is therefore also in a range from 0.25 to 0.8 g/cm³,preferably 0.25 to 0.6 g/cm³, in particular <0.5 g/cm³ for theseembodiments.

The cover layer may possibly additionally contain typical additives suchas stabilizers, neutralization agents, antiblocking agents, lubricants,antistatic agents, etc., in the particular effective quantities.

The porous film for the use according to the present invention ispreferably produced according to extrusion methods or coextrusionmethods known per se.

In the scope of this method, the polypropylene, which is admixed withβ-nucleating agent, is melted in an extruder and extruded through asheet die onto a drawing-off roll, on which the melt solidifies whileforming the β-crystallite. In the case of the two-layered embodiment,the corresponding coextrusion is performed together with the coverlayer. The cooling temperatures and cooling times are selected in such away that the highest possible proportion of β-crystalline polypropylenearises in the precursor film. This precursor film having a highproportion of β-crystalline polypropylene is subsequently biaxiallystretched in such a way that a conversion of the β-crystallite intoalpha polypropylene and a delimitation of the network structure occursduring the stretching. The biaxially stretched film is subsequentlythermally fixed and possibly corona, plasma, or flame treated on asurface.

The biaxial stretching (orienting) is generally performed sequentially,the stretching preferably first being performed longitudinally (in themachine direction) and then transversely (perpendicular to the machinedirection).

The drawing-off roll or rolls are kept at a temperature of 60 to 130°C., preferably 80 to 120° C., to encourage the formation of a highproportion of β-crystalline polypropylene.

During the stretching in the longitudinal direction, the temperature isless than 140° C., preferably 90 to 125° C. The stretching ratio is in arange from 2:1 to 5:1. The stretching in the transverse direction isperformed at a temperature of greater than 140° C., preferably at 145 to160° C. The transverse stretching ratio is in a range from 3:1 to 6:1.

The longitudinal stretching is expediently performed with the aid of tworolls running at different speeds corresponding to the desiredstretching ratio and the transverse stretching is performed with the aidof a corresponding tenting frame.

The biaxial stretching of the film is generally followed by its thermalfixing (heat treatment), the film being held approximately 0.5 to 10seconds long at a temperature of 110 to 150° C. The film is subsequentlywound up in a typical way using a winding unit.

Preferably, as noted above, a surface of the film is typically corona,plasma, or flame treated according to one of the known methods after thebiaxial stretching.

For the alternative corona treatment, the film is guided between twoconductor elements used as electrodes, such a high voltage, usually ACvoltage (approximately 10,000 V and 10,000 Hz) being applied between theelectrodes that spray or corona discharges may occur. The air above thefilm surface is ionized by the spray or corona discharge and reacts withthe molecules of the film surface so that polar intercalations arise inthe essentially nonpolar polymer matrix. The treatment intensities arein the typical scope, 38 to 45 mN/m being preferred.

A film having a porous layer is obtained according to this method. Thefilm is distinguished overall by a white or opaque appearance. Theporous layer has a net-like structure (see FIGS. 1 a and 1 b), which ispermeable to gases. The gas permeability of the porous layer may bedetermined by the Gurley value, for example, which indicates how longthe passage of 100 cm³ of air takes through the single-layer film underdefined conditions.

It has been found that higher gas permeabilities, i.e., accordinglylower Gurley values, are particularly advantageous in regard to bubblingand adhesion. Therefore, films which have a microporous layer having aGurley value of >50 to 5000 seconds are preferred. Surprisingly,however, very good results have also been found using comparativelydenser films in which the Gurley value is above 5000. It has been foundthat Gurley values may be up to 300,000 seconds and nonetheless therequired good adhesion and freedom from bubbles may be achieved. It issurprising that films having comparatively lower permeabilities areequally well suitable, because the good adhesion and freedom frombubbles was originally attributed to good ventilation through the porousstructure of the layer. It was thus to be expected that a film havinglower gas permeabilities of >5000 Gurley would be less suitable for thedeep drawing application. Surprisingly, this is not the case.

Therefore, embodiments having Gurley values of the porous layer of >5000to 300,000 Gurley, preferably 8000 to 250,000 Gurley are also preferred.These embodiments may be produced at higher production speeds andtherefore have significant economic advantages in relation to the highlypermeable embodiments. In particular, the cooling times on thedrawing-off roll may be shortened here, by which the production speedsmay be significantly increased.

According to the present invention, the film is used as a label in deepdrawing. In suitable deep-drawing methods, thick films made ofthermoplastic polymers are plastically molded at elevated temperatureunder the effect of pneumatic forces or by the mechanical action ofmolds. The plastic molding using pneumatic forces may be performed bypartial vacuum (deep drawing) or excess pressure, i.e., compressed air.Methods of this type are known in the prior art and are referred to inEnglish as “thermoforming”. The methods and their embodiments aredescribed in detail, for example, in Rosato's Plastics Encyclopedia andDictionary, pages 755 through 766, to which reference is herebyexpressly made.

Plastic molding under the effect of pneumatic forces is performed, forexample, using partial vacuum after the film to be deep drawn hastypically been pre-shaped using a top ram. Before the actual deepdrawing, the label film is laid in the mold body and the deep drawingfilm is laid over it in such a way that the mold body is sealedairtight. A partial vacuum or vacuum is applied to the mold body in asuitable way. Because of the pressure differential, a suction acts onthe deep drawing film. A heating element is attached above the filmsurface and heats the film until it deforms in the direction of the moldbody. Temperature and partial vacuum are selected in the process in sucha way that the film presses in a formfitting way against the mold bodyhaving the inserted label and bonds to the label. After removal of thepressure differential and cooling, the labeled, deep-drawn container maybe removed.

Various embodiments of the deep-drawing methods are shown as examples inFIG. 3 and schematically show devices for deep drawing. For the methodsfor deep drawing are illustrated in FIG. 4. In principle, any arbitrarysuitable molds which may be evacuated and possibly molding tools may beused in deep drawing.

The following measurement methods were used for characterizing the rawmaterials and the films:

Melt-Flow Index

The melt-flow index of the propylene polymers was measured at 2.16 kgload and 230° C. according to DIN 53 735 and at 190° C. and 2.16 kg forpolyethylenes.

Melting Points

DSC measurement, maxima of the melting curve, heating speed 20 K/minute,

β-Crystal Content

The DSC methods were used to determine the β-crystalline proportion (forexample, in the precursor film) in polypropylene.

The characterization using DSC is described in J. o. Appl. PolymerScience, Vol. 74, pages: 2357-2368, 1999 by Varga and performed asfollows: the sample having the β-nucleator added is first heated in theDSC at a heating rate of 20° C./minute to 220° C. and melted (firstheating). It is then cooled at a cooling rate of 10° C./minute to 100°C., before it is melted again at a heating rate of 10° C./minute (secondheating). During the second heating, the degree of crystallinityK_(β,DSC) is determined from the ratio of the melt enthalpies of theβ-crystalline phase (H_(β)) to the sum of the melt enthalpies of β- andα-crystalline phases (H_(β)+H_(α))

Density

The density is determined in accordance with DIN 53 479, method A.

Porosity

The porosity is calculated from the densities of the non-voided PP(δ_(PP)) and the density of the voided PP (δ_(PPV)), as follows:Porosity [%]=100*(1−[δ_(PPV)/δ_(PP)])Permeability (Gurley Value)

The permeability of the label films was measured using the Gurley tester4110, in accordance with ASTM D 726-58. The time which 100 cm³ of airrequired to permeate through the label area of 1 in.² (6.452 cm²) wasdetermined. The pressure differential over the film corresponds to thepressure of a water column of 12.4 cm in height. The time required thencorresponds to the Gurley value.

The present invention will now be explained by the following examples.

Example 1

A single layer film was extruded from a sheet die at an extrusiontemperature of 245° C. according to the extrusion method. The film hadthe following composition:

approximately 50 weight-percent propylene homopolymer (PP) having an n-heptane soluble proportion of 4.5 weight-percent (in relation to 100%PP) and a melting point of 165° C.; and a melt-flow index of 3.2 g/10minutes at 230° C. and 2.16 kg load (DIN 53 735) and approximately 49.9weight-percent propylene-ethylene block copolymer having an ethyleneproportion of approximately 5 weight-percent in relation to the blockcopolymer and an MFI (230° C. and 2.16 kg) of 6 g/10 minutes 0.1weight-percent calcium pimelate as a β- nucleating agent

The film contained additional stabilizers and neutralization agents intypical quantities.

The molten polymer mixture was drawn off after the extrusion via a firstdrawing-off roll and a further roll trio and solidified, subsequentlylongitudinally stretched, transversely stretched, and fixed, thefollowing specific conditions having been selected:

extrusion: extrusion temperature 245° C. cooling roll: temperature 125°C. drawing-off speed: 1.5 m/minute (dwell time on the drawing-off roll:55 seconds) longitudinal stretching: stretching roll T = 90° C.longitudinal stretching by a factor of 4 transverse stretching: heatingpanels T = 145° C. stretching panels T = 145° C. transverse stretchingby a factor of 4

The porous film thus produced was approximately 80 μm thick and had adensity of 0.35 g/cm³ and displayed a uniform white-opaque appearance.The porosity was 56% and the Gurley value was 1040 seconds.

Example 2

A film was described as produced in Example 1. In contrast to Example 1,0.3 weight-percent, in relation to the weight of the layer, of adicarboxamide was now used as the β-nucleating agent. The porous filmthus produced was approximately 70 μm thick and had a density of 0.40g/cm³ and displayed a uniform white-opaque appearance. The porosity was51% and the Gurley value was 1200 seconds.

Example 3

A film was produced as described in Example 1. The composition was notchanged. In contrast to Example 1, a higher drawing-off speed wasselected in the production: 3 m/minute (dwell time on the drawing-offroll: 27 seconds) and a drawing-off temperature of 120° C. was set. Theporous film thus produced was approximately 60 μm thick and had adensity of 0.5 g/cm³ and displayed a uniform white-opaque appearance.The porosity was 41% and the Gurley value was 36,000 seconds.

Example 4

A film was produced as described in Example 1. The composition was notchanged. In contrast to Example 1, a higher drawing-off speed wasselected of 5 m/minute (dwell time on the drawing-off roll: 17 seconds)and a drawing-off temperature of 115° C. was set in the production. Theporous film thus produced was approximately 90 μm thick and had adensity of 0.5 g/cm³ and displayed a uniform white-opaque appearance.

The porosity was 42% and the Gurley value was 170,000 seconds.

Comparative Example 1

An opaque three-layer film having a layer structure ABC having a totalthickness of 80 μm was produced by coextrusion and subsequentstep-by-step orientation in the longitudinal and transverse directions.The cover layers each had a thickness of 0.6 μm.

Base layer B (=vacuole-containing layer):

93 weight-percent propylene homopolymer having a melting point of 165°C.

7.0 weight-percent CaCO₃ of the Millicarb type having a mean diameter of3 μm

Cover Layer A

-   99.67 weight-percent random ethylene-propylene copolymer having a C₂    content of 3.5 weight-percent-   0.33 weight-percent SiO₂ as an antiblocking agent having a mean    diameter of 2 μm-   cover layer B like cover layer A

The production conditions in the individual method steps were:

extrusion temperatures 280° C. temperature of the drawing-off roll:  30°C. longitudinal stretching: temperature: 122° C. longitudinal stretchingratio: 6.0 transverse stretching: temperature: 155° C. transversestretching ratio: 8.0 fixing: temperature: 140° C. convergence: 15%

In this way, an opaque, vacuole-containing film having a density of 0.6g/cm³ was obtained. The film was not porous, a Gurley value thereforecould not be determined for this film.

Use According to the Present Invention

The films according to the examples and the comparative example wereused as label films in deep drawing a margarine tub. For this purpose,the labels were cut into cross shapes, the blanks were stacked andprovided in a magazine to the deep drawing system. The deep drawingsystem was equipped with a top ram as a molding aid. The labels wereremoved from the magazine by suction and folded in such a way that thefaces of the cross-shaped label covered the later side walls of thecontainer. The folded label was laid in the mold, placed using anauxiliary core, and held by suction.

A 600 μm thick PP deep-drawing film was heated using IR radiators upinto the range of its plastic deformability (>165° C.). By lowering thetop ram and applying a vacuum through holes in the wall of the mold, thedeep drawing film was deformed, so that it bonded with the insertedlabel.

The labeled container was checked in regard to adhesion and appearance.It was shown that the film of Comparative Example 1 had significantbubbling between film and container wall and thus impaired adhesion.

The microporous films according to Examples 1 through 4 displayed ahomogeneous appearance of the label surface without bubbling or othervisual flaws, as well as good adhesion of the label to the containersurface. Surprisingly, the labeled containers did not differ in theirvisual quality, although the films according to Examples 3 and 4 hadsignificantly lower gas permeabilities than typical porous films.

1. A biaxially oriented film having a microporous layer, which comprisesa propylene polymer and at least one β-nucleating agent and whosemicroporosity is generated by converting β-crystalline polypropyleneduring stretching of the film, wherein the porous layer has a Gurleyvalue in a range from 100,000 to 500,000 seconds.
 2. The film accordingto claim 1, wherein the density of the film is in a range from 0.2 to0.80 g/cm³.
 3. The film according to claim 2, wherein the microporouslayer contains a mixture of propylene homopolymer and propylene blockcopolymer and the ratio is in a range from 90:10 to 10:90.
 4. The filmaccording to claim 3, wherein the microporous layer contains 0.001weight-percent to 5 weight-percent β-nucleating agent in relation to theweight of the β-nucleated layer.
 5. The film according to claim 4,wherein the nucleating agent is a calcium salt of pimelic acid orsuberic acid or a carboxamide.
 6. The film according to claim 5, whereinthe microporous layer is provided with a cover layer on one side.
 7. Thefilm according to claim 6, wherein the film is produced according to thetentering method and the drawing-off roll temperature is in a range from60 to 130° C.
 8. A label for a container which comprises the film asclaimed in claim
 7. 9. The label according to claim 8, wherein theapplied label does not have an orange peel effect.
 10. A label for acontainer which comprises the film as claimed in claim
 2. 11. The filmaccording to claim 1, wherein the microporous layer contains a propylenehomopolymer and/or a propylene block copolymer.
 12. The film accordingto claim 1, wherein the microporous layer contains a mixture ofpropylene homopolymer and propylene block copolymer and the ratio is ina range from 90:10 to 10:90.
 13. A label for a container which comprisesthe film as claimed in claim
 12. 14. The film according to claim 1,wherein the microporous layer contains 0.001 weight-percent to 5weight-percent β-nucleating agent in relation to the weight of theβ-nucleated layer.
 15. The film according to claim 14, wherein thenucleating agent is a calcium salt of pimelic acid or suberie acid or acarboxamide.
 16. A label for a container which comprises the film asclaimed in claim
 14. 17. The label according to claim 16, wherein theapplied label does not have an orange peel effect.
 18. The filmaccording to claim 1, wherein the microporous layer is provided with acover laser on one side.
 19. The film according to claim 1, wherein thefilm is produced according to the tentering method and the drawing-offroll temperature is in a range from 60 to 130° C.
 20. A label for acontainer which comprises the film as claimed in claim 1.